what is the use of !(value) in c++

Question

5.00/5 (1 vote)

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I have very simple problem.
I have a program in which some one used ! like this

C++

int main()
{
    int a;
    a=5;
    if (!a==0)
        cout<<"yes";
    else
        cout<<"no";
    return 0;
}

can any one tell me what is used of '!' in this program?

Posted 26-Oct-11 0:10am

Neeil

Updated 30-Oct-11 22:27pm

Malli_S

v4

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Comments

Nagy Vilmos 26-Oct-11 8:13am

Formatted snippet.

5 solutions

Solution 6

Don't write codes like that.

Do like this:

if( !(a==0))
{
...
}

or

if((!a) == 0)
{
...
}

I think it's a better way.

Posted 30-Oct-11 19:57pm

sky163@codeproject

Solution 4

It's also used when the successful return code from an API is zero (0) and the error return code can have different values (1 for error X, 2 for error Y, .... )

(I think I've seen this in some win32 API).

for example ...

C++

/// \return zero on success.
/// \return 1 allocation error.  
/// \return 2 division by zero.
int f( );

int f( )
{
  if ( ... )
  {
    // cannot allocate.
    return 1;
  }

  if ( ... )
  {
    // division by zero
    return 2;
  }
  // success
  return 0;
}

Posted 26-Oct-11 2:59am

Maximilien

Comments

Chuck O'Toole 26-Oct-11 14:31pm

that would be a good example except is hasn't got a single ! in it :)

Maximilien 26-Oct-11 15:05pm

oh!!!

elgaabeb 27-Oct-11 5:32am

sorry but i can't see any convenience ! :)

Solution 5

Hi there,

This syntaxe of logical NOT is used to check if a variable is "alive" or not.
When a variable is initialized then it is considered as alive by the compiler.
It can then participate in any necessary operation or treatment.

(!a==0) is equivalent to ((!a) == 0)

i'll rewrite the message to be more significant for you example :

C#

int a;
a=5;
if (!a==0)
    cout<<"yes VARIABLE IS ALIVE";
else
    cout<<"no VARIABLE IS NOT ALIVE";

Check this link for more explanation paragraph "The Logical Not Operator !":

http://www.functionx.com/cpp/Lesson08.htm[^]

best regards !

Posted 26-Oct-11 23:18pm

elgaabeb

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Nagy Vilmos · Accepted Answer · 2011-10-26T02:17:00

Solution 3

!a==b is not the same as a!=b. Check you precedence, as said above, this is equal to (!a) == (b) so first a is negated then compared to b.

Posted 26-Oct-11 2:17am

Nagy Vilmos

Comments

elgaabeb 27-Oct-11 5:26am

This is not a bitwise negation ... it is equivalent to (!a)== 0 ... this is another usage of logical negation to check if a variable is alive and reachable by the compiler.

Nagy Vilmos 27-Oct-11 7:05am

"it is equivalent to (!a)== 0" - that is exactly what I said.

eli551 25-Mar-12 5:06am

i have a simple problem too. how can check equality twain sentence?

eli551 31-Mar-12 12:18pm

i want check this:
if( s == roze smith )
{
-------;
}
what should i write?

Andrew Brock · Accepted Answer · 2011-10-26T00:13:00

Solution 1

! is the not operator.
!a==0 is read as "not a is equal to 0". This is a bad usage of it, but is valid, typically this would be written as a!=0, read "a is not equal to 0"

EDIT:
in this case the program prints "no" if a is 0, otherwise prints "yes".

Also, you misspelt return.

Posted 26-Oct-11 0:13am

Andrew Brock

Updated 26-Oct-11 0:17am

v2

Comments

Espen Harlinn 26-Oct-11 6:23am

http://en.cppreference.com/w/cpp/language/operator_precedence
(!a==0) => ((!a) == 0) ! has higher precedence than ==
The case (!a==2) can not be rewritten as a != 2

RaisKazi 26-Oct-11 6:29am

That link is nice. This question was misunderstood by me initially.:)

Andrew Brock 26-Oct-11 6:35am

I should have remembered that.
By luck, my answer is still correct because he is checking for equivalence to 0, although the explanation is a quite misleading.

Espen Harlinn 26-Oct-11 6:45am

But you where right in this case, so my 5 :)

RaisKazi 26-Oct-11 6:23am

My 5!

Albert Holguin 26-Oct-11 15:48pm

bad usage indeed... +4

elgaabeb 27-Oct-11 5:24am

This is not a bad usage ... it is well a right syntax (!a==b) ... this is another usage of logical negation to check if a variable is alive.